This invention relates to a thermal recording stylus, and more particularly to an improved nib for a thermal recording stylus.
Thermal recording on heat sensitive paper has been in use in Telautograph systems for many years. These systems are designed to transmit graphic information, written or drawn by an operator, or otherwise produced by a machine, at a transmitter, and to accurately reproduce the graphic information on heat sensitive paper at a receiver. An advantage of thermal recording over ink recording is that an unattended receiver may be used with much greater confidence that the stylus is functional.
Receivers that use an ink stylus, such as a ball-point pen, suffer from many problems related to just the stylus. First there is the problem of running out of ink. Then there is the problem of the ink drying at the nib, thus clogging the free flow of ink, and in the case of ball-point pens, causing the ball to stick or otherwise "skip," particularly when used infrequently.
This is not to say that thermal recording does not have its own problems. However, such problems may be readily solved. For example, there is a problem of fast start-up, but that problem can be solved by proper design of the nib. An excellent solution disclosed in U.S. Pat. No. 4,027,311 is forming a narrow strip of resistive metal into a nib over the tip of a ceramic stylus, and applying sufficient electrical current through the strip to heat the nib to the desired temperature.
Although this resistive metal nib over a ceramic stylus has been used very successfully, there is the problem of wear on the nib. The thermal paper, like most paper, is abrasive, so it tends to grind the nib material away. This decreases the cross-sectional area of the strip at the nib, thereby increasing resistivity and consequently increasing the temperature of the nib. However, this is not a critical problem since very good thermal recording is possible over a wide temperature range. The more critical problem is the nib wearing completely through and breaking open, in which case current flow is interrupted and recording stops.